66 



ENDOGENOUS OR MONOCOTYLEDONOUS STEM. 



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stems was increased by additions to the centre, so that the first- 

 formed fibres were gradually pushed towards the circumference by 

 those which succeeded them, in the manner represented in Fig. 133, 



1 : hence the term Endogenous 

 (eVSok, within, and yiivdnv, to pro- 

 duce), meaning internal growth. 

 But Mohl has shown that this 

 is not strictly correct. For 

 although the fibres connected 

 with the leaves, in the first in- 

 stance, are directed towards the 

 centre, and are therefore always 

 internal to those previously 

 formed, yet, when they are traced 

 downwards, they are found not 

 to continue in a parallel direc- 

 tion, but to arch outwards, so as 

 ultimately to reach the circum- 

 ference. Hence, the newly-form- 

 ed fibres really become external 

 at the base, although internal 

 above. On making a vertical 

 section of an endogenous stem, 

 as of a Palm, there is observed 

 an interlacing of fibres, simUar 

 to what is represented iu Fig. 

 133, 2, where the four vascular 

 bundles, abed, are first direct- 

 ed towards the centre, and then 

 curve outwards towards the cir- 

 cumference, so that those last 

 formed ultimately become ex- 

 ternal. The term Endogenous 

 wUl, therefore, only apply strict- 

 ly to the fibres at the early part 

 of their course. Of late years, 

 the terms Endogenous and Exo- 



Kg. 133. 



genous have been discarded by many writers, the terms Mono- 

 cotyledonous and Dicotyledonous being substituted. The .true dis- 

 tinction between Exogenous and Endogenous stems is, that in the 

 former the woody or vascular bundles increase indefinitely at their 



Fig. 133. DiagraniB illustrating the arrangement of four pairs of vascular bundles (a a, 

 hb, cc, d d), in endogenous stems. 1. According to the old idea of internal development 

 throughout the stem. 2. According to the view of Mohl, -who has shown that the flbres 

 interlace, and that those which are at first internal become external, lower down. 



